PROTEINS OF NEWLY ISOLATED MUTANTS AND THE AMINO-TERMINAL PROLINE AREESSENTIAL FOR UBIQUITIN-PROTEASOME-CATALYZED CATABOLITE DEGRADATION OF FRUCTOSE-1,6-BISPHOSPHATASE OF SACCHAROMYCES-CEREVISIAE

Addition of glucose to cells of the yeast Saccharomyces cerevisiae gro wing on a non-fermentable carbon source leads to selective and rapid d egradation of fructose-1,6-bisphosphatase, This so called catabolite i nactivation of the enzyme is brought about by the ubiquitin-proteasome system. To identify additional components of the catabolite inactivat ion machinery, we isolated three mutant strains, gid1, gid2, and gid3, defective in glucose-induced degradation of fructose-1,6-bisphosphata se, All mutant strains show in addition a defect in catabolite inactiv ation of three other gluconeogenic enzymes: cytosolic malate dehydroge nase, isocitrate lyase, and phosphoenolpyruvate carboxykinase, These f indings indicate a common mechanism for the inactivation of all four e nzymes. The mutants were also impaired in degradation of short-lived N -end rule substrates, which are degraded via the ubiquitin-proteasome system. Site-directed mutagenesis of the amino-terminal proline residu e yielded fructose-1,6-bisphosphatase forms that were no longer degrad ed via the ubiquitin-proteasome pathway. All amino termini other than proline made fructose-1,6-bisphosphatase inaccessible to degradation. However, the exchange of the amino-terminal proline had no effect on t he phosphorylation of the mutated enzyme. Our findings suggest an esse ntial function of the amino-terminal proline residue for the degradati on process of fructose-1,6-bisphosphatase. Phosphorylation of the enzy me was not necessary for degradation to occur.